title: Visualization of BRI1 and BAK1(SERK3) membrane receptor heterooligomers during brassinosteroid signaling.
abstract: The leucine-rich repeat receptor-like kinase BRASSINOSTEROID-INSENSITIVE1 (BRI1) is the main ligand-perceiving receptor for brassinosteroids (BRs) in Arabidopsis (Arabidopsis thaliana). Binding of BRs to the ectodomain of plasma membrane (PM)-located BRI1 receptors initiates an intracellular signal transduction cascade that influences various aspects of plant growth and development. Even though the major components of BR signaling have been revealed and the PM was identified as the main site of BRI1 signaling activity, the very first steps of signal transmission are still elusive. Recently, it was shown that the initiation of BR signal transduction requires the interaction of BRI1 with its SOMATIC EMBRYOGENESIS RECEPTOR-LIKE KINASE (SERK) coreceptors. In addition, the resolved structure of the BRI1 ectodomain suggested that BRI1-ASSOCIATED KINASE1 [BAK1](SERK3) may constitute a component of the ligand-perceiving receptor complex. Therefore, we investigated the spatial correlation between BRI1 and BAK1(SERK3) in the natural habitat of both leucine-rich repeat receptor-like kinases using comparative colocalization analysis and fluorescence lifetime imaging microscopy. We show that activation of BR signaling by exogenous ligand application resulted in both elevated colocalization between BRI1 and BAK1(SERK3) and an about 50% increase of receptor heterooligomerization in the PM of live Arabidopsis root epidermal cells. However, large populations of BRI1 and BAK1(SERK3) colocalized independently of BRs. Moreover, we could visualize that approximately 7% of the BRI1 PM pool constitutively heterooligomerizes with BAK1(SERK3) in live root cells. We propose that only small populations of PM-located BRI1 and BAK1(SERK3) receptors participate in active BR signaling and that the initiation of downstream signal transduction involves preassembled BRI1-BAK1(SERK3) heterooligomers.
authors.affiliation: Laboratory of Biochemistry , Department of Agrotechnology and Food Sciences, 6703 HA Wageningen, The Netherlands.
authors.forename: Christoph A
authors.initials: CA
authors.lastname: Bücherl
authors.forename: G Wilma
authors.initials: GW
authors.lastname: van Esse
authors.forename: Alex
authors.initials: A
authors.lastname: Kruis
authors.forename: Jeroen
authors.initials: J
authors.lastname: Luchtenberg
authors.forename: Adrie H
authors.initials: AH
authors.lastname: Westphal
authors.forename: José
authors.initials: J
authors.lastname: Aker
authors.forename: Arie
authors.initials: A
authors.lastname: van Hoek
authors.forename: Catherine
authors.initials: C
authors.lastname: Albrecht
authors.forename: Jan Willem
authors.initials: JW
authors.lastname: Borst
authors.forename: Sacco C
authors.initials: SC
authors.lastname: de Vries
mesh_terms: D017360:Arabidopsis / Q000187:drug effects / Q000235:genetics / Q000378:metabolism*; D029681:Arabidopsis Proteins / Q000235:genetics / Q000378:metabolism*; D060406:Brassinosteroids / Q000378:metabolism*; D020126:Brefeldin A / Q000378:metabolism / Q000494:pharmacology; D002462:Cell Membrane / Q000378:metabolism; D008856:Microscopy, Fluorescence / Q000379:methods; D018517:Plant Roots / Q000166:cytology / Q000378:metabolism; D030821:Plants, Genetically Modified; D011494:Protein Kinases / Q000235:genetics / Q000378:metabolism*; D055503:Protein Multimerization; D017346:Protein Serine-Threonine Kinases / Q000235:genetics / Q000378:metabolism*; D015398:Signal Transduction; D014230:Triazoles / Q000494:pharmacology
chemical_list: D029681:Arabidopsis Proteins; D060406:Brassinosteroids; D014230:Triazoles; D020126:Brefeldin A; D011494:Protein Kinases; C463660:BAK1 protein, Arabidopsis; C108270:BRI1 protein, Arabidopsis; D017346:Protein Serine-Threonine Kinases; C412342:brassinazole